1. Field of the Invention
The present invention relates to red emitting phosphor for cathode ray tubes such as color Braun tubes and cathode ray tubes using the same.
2. Description of the Related Art
A phosphor layer of a color cathode ray tube is generally fabricated in the following way. That is, first, into water solution that contains polyvinyl alcohol (PVA), ammonium dichromate and a surfactant, phosphor is dispersed to prepare a phosphor slurry. This phosphor slurry is coated on a glass panel to form a phosphor layer. Then, onto this phosphor layer, ultra-violet rays are irradiated through a shadow mask to cure the PVA of irradiated portion. By developing, the phosphor layer other than the cured portion is removed. In this way, the phosphor layer is formed in stripe or in dot.
Upon forming a phosphor layer by the use of the aforementioned coating method, the following characteristics are demanded for the phosphor. That is, the phosphor (1) is capable of forming a phosphor layer of closely disposed stripe or dot, (2) does not mingle with another phosphor of different color, (3) sticks strongly to a panel, and (4) can form a phosphor layer of sufficient thickness. Further, in the case of pigmented phosphor being employed, the pigment is required not to peel off the phosphor in the slurry solution.
So far, in order to satisfy the aforementioned requirement on phosphor, a variety of kinds of surface treatments of phosphor have been implemented to improve and develop. For instance, Japanese Patent Laid Open Publication No. (KOKAI) Sho 54-102299 and Japanese Patent Publication (KOKOKU) No. Sho 59-8310 disclose methods in which by letting contact the pigmented phosphor with a solution of water soluble organic compound, dispersion ability of the phosphor is improved.
In addition, Japanese Patent Publication (KOKOKU) Nos. Sho 60-21675, Sho 61-46512 and Sho 62-39186 disclose method that improves dispersion property of phosphor by sticking an inorganic compound on the surface of phosphor. Further, Japanese Patent Laid-Open Publication (KOKAI) No. Hei 2-178387 discloses a method in which by sticking an inorganic compound and an organic compound on the surface of phosphor, settling property of phosphor in a slurry solution is improved.
As described above, so far, by the use of the surface treatment in which an inorganic compound or an organic compound is stuck on the phosphor, quality of the phosphor layer has been improved to some degree. However, accompanying the recent enlargement of the computer market, color cathode ray tubes are demanded to satisfy higher performance that the computer display terminal demands. Accordingly, the phosphor layer thereof is also demanded to satisfy requirement of higher performance. In particular, demand for higher contrast is strong. In addition, also in cathode ray tubes for color television, upon making the screen larger and finer, the phosphor layer of higher quality, particularly of higher contrast, is being demanded.
As a method for attaining the higher contrast, an improvement of the brightness of phosphor layer can be cited. In order to improve the brightness of the phosphor layer, the phosphor itself is required to improve the brightness thereof, or by forming a denser phosphor layer than ever, the phosphor layer can improve the brightness thereof.
Now, the present inventors have paid attention to a method that materialize densification of the phosphor layer to improve the brightness thereof and accordingly the contrast of color cathode ray tubes. With the above intention, the inventors have studied in detail the quality of the phosphor layer of the existing color cathode ray tube. As a result of this, it is found that denseness of the red emitting phosphor layer is remarkably poor in contrast with those of green and blue emitting phosphor layers. The low denseness of the red emitting phosphor layer does have no relation with the order by which each phosphor of three colors is coated.
Thus, through improvement of the denseness of the red emitting phosphor layer, the brightness of the red emitting phosphor layer can be improved, as a result of this higher contrast of a color cathode ray tube can be realized. However, the existing red emitting phosphor can not form such dense red emitting phosphor layer.
Further, Japanese Patent Laid-Open Publication (KOKAI) No. Hei 3-220286 discloses a red emitting phosphor (phosphor that has yttrium oxysulfide as a phosphor base material) of which content of particles of particle diameter coarser than 8.01 .mu.m is 10% or less. However, only by reducing the ratio of the coarser particles, the denseness of the red emitting phosphor layer can not be sufficiently heightened.
Further, in Japanese Patent Laid-Open Publication (KOKAI) No. Hei 8-41453, a fabrication method is disclosed in which rare earth oxide raw material is mixed with alkali metal carbonate, aluminum compound and sulfur raw material, and the mixture thereof is fired to fabricate a rare earth oxysulfide phosphor. In the embodiment of this publication, as criteria of particle size distribution, -log (d.sub.84 /d.sub.50) and +log (d.sub.16 /d.sub.50) are disclosed. However, the red emitting phosphor having the disclosed particle distribution can not sufficiently improve the denseness of the phosphor layer.